Modern turbofan engines typically comprise assemblies having inner and outer casings or ducts. Multiple casings are required because these engines make use of an inner airflow, that passes through a compressor and a turbine, and an outer airflow, that passes through a fan or low pressure compressor and then bypasses the engine turbine. These two airflows are typically reunited in the engine exhaust. In order to support these two separate airstreams inner and outer casings are maintained through a large portion of the engine.
The outer casing, or duct, is generally supported by links connecting to the inner casing or the engine frame. One method of supporting the outer casing in conventional engines is by using several pin ended links. Such links (or alternatively turnbuckles) require mounting brackets at both their inner end, which is connected to the turbine frame, and their outer end, which is connected to the outer casing duct.
These links have been provided to allow for a limited amount of relative movement between the turbine frame and the outer casing duct. Limited movement must be allowed to accommodate differential thermal expansion of the outer casing duct relative to the turbine frame. The turbine frame generally reaches much higher temperatures during engine operation than the outer casing duct, since the turbine airflow is quite hot after passing through the combustor. The temperature difference between the inner and outer airflows and the different geometry of the ducts results in differential thermal expansion.
This type of mounting arrangement has been successfully used for many years in conventional turbofan aircraft engines. The incorporation of new materials into aircraft engine ducting has created new linkage problems that can not be easily accommodated by conventional assemblies. New exhaust casing duct designs and geometries have made the incorporation of conventional link support devices more difficult. Further, newer engines have been designed to run at elevated operating temperatures, as a result, relative thermal expansion is a larger problem than in older engines.
Many composite materials such as polymide graphite, carbon-carbon and graphite epoxys, which have a much lower coefficient of thermal expansion than the conventional metallic casings, are now being incorporated into outer casing ducts because of their high strength and low weight. Metallic core engine turbine casings and frames typically undergo expansion in the aft and radial direction to a larger degree than composite fan casings. A conventional pin ended link support system between a turbine frame and a composite outer casing duct will operate in compression in all engine operating conditions with greater loads superimposed during flight maneuver accelerations. Excessive compression loads caused by differential thermal expansion result in hardware cracking and deformation.
A further problem with conventional link support systems is the need to attach and detach each link during every installation and removal of the outer duct. This is a tedious operation that is required each time maintenance is performed on the aft engine core. It would be advantageous if a time saving means was devised for removing the outer duct in order to speed up and simplify engine maintenance.
A need therefore exists for an improved outer casing duct support mechanism that can accommodate use of composite materials in new technology turbofan engines.
A further need exists for a turbine outer case mounting device which will facilitate easy removal of the engine outer casing duct.